Si3N4 insulator material for corona discharge igniter systems

ABSTRACT

A silicon nitride material is disclosed which has properties beneficial for efficient operation of a corona discharge igniter system in an internal combustion gas engine.

FIELD

The present invention pertains generally to the field of technicalceramic materials and more specifically to a high performance monolithicsilicon nitride material used as a ceramic insulator in corona dischargeigniter systems for internal combustion engines and gas turbines.

BACKGROUND

Due to the recently rising prices of oil, as well as legislation foremission control, increases in fuel efficiency in internal combustionengines have become extremely important. Governments in many countrieshave mandated automobile manufacturers to reach increased gas mileagefor vehicle models manufactured in the near future.

One way to increase the fuel efficiency is to change the current stateof the art of spark plugs which ignite the fuel in the internalcombustion engine by generating a spark (plasma) between narrow gapelectrodes separated by an air-fuel mixture, by more energy efficientcorona discharge igniter (CDI) systems. In a CDI system, instead of ashort duration spark, a steady corona discharge is generated betweenelectrodes and around the ceramic insulator tip using a radio frequencyelectromagnetic field. The generation of a corona discharge does notrequire a dielectric breakdown of the air-gas mixture to occur to ignitethe fuel. The corona is therefore generated at lower voltages thanrequired for a spark. The corona generates a steady fuel burning frontthat is easier to control and increases the fuel efficiency. It alsoallows ignition of lean fuel mixtures that burn cleaner but aredifficult to ignite using spark-plugs.

One CDI system is described in U.S. Pat. No. 6,883,507 (Freen), and itspecifies that a feed-thru insulator is used for the system to work. Thepatent advises the use of BN (boron nitride) for this purpose (line35-36, column 6), and does not provide additional guidance for thenature of insulator material.

U.S. Publication No. 2011/0175691 A1 (Smith et al) describes a compactelectromagnetic device generating a corona discharge in a coaxialresonating cavity that can be used to ignite combustible materials incombustion engines. In their invention, an insulated guide is requiredwhich is shown in FIG. 5 (item 510) and FIG. 6 (item 610), however thematerial used is not specified in any way in the disclosure.

U.S. Publication No. 2011/0247579 A1 (Hampton et al) describes a coronaigniter with an enhancing electrode tip composed of metal shapes at thetip of the insulator. The type of insulator is only described as “aninsulator that surrounds the electrode body portion and extends from theinsulator tip to to insulator upper end” (paragraph [0086]).

U.S. Publication No. 2010/0282197 A1 (Permuy et al) describes in detailthe preferred shapes of the feed-thru insulator necessary for this CDIsystem and specifies that the insulator should be a ceramic material.Same application notes (first three lines of paragraph [0006]) that,although BN is suggested in U.S. Pat. No. 6,883,507 for the insulatordue to its high dielectric breakdown strength and a low dielectricconstant, that the material is very soft, expensive and difficult toform into required insulator shapes. It is also noted that relativepermittivity (i.e. dielectric constant) should be low for the materialto have a high dielectric strength (paragraph [0007]), but offers nospecific insulator materials that have these properties.

U.S. Publication No. 2010/0175655 A1 (Lykowski et al) describes furtherthat the ceramic insulator (which can be combined with a non-ceramicinsulator) is an aluminum or silicon (paragraph [0037] lines 2 and 4)containing oxide and or nitride based ceramic with up to 5% additions ofcalcium oxide, magnesium oxide, zirconium oxide, boron oxide or boronnitride additions to alumina or silica (paragraph [0039]). Lykowsky etal also disclose that the desired dielectric strength of the ceramicinsulator should be above 15 kV/mm (or more preferably 17 kV/mm or aboveand most preferably above 19 kV/mm) (paragraph [0051]). Additionally,the application describes that the ceramic material should have amodulus of rupture strength (MOR) of at least 100, 200 or 400 MPa (inincreasing order of preference), low dielectric constant at 1 MHz (lowerthan 9-paragraph [0069]), and low loss tangent (most preferably lessthan 0.005 at 1 MHz-paragraph [0053]). This patent application does notprovide any examples of inventive compositions their invention requires.Specific compositions involving silicon nitride, how they can beproduced or what they consist of when produced are also not disclosed.The inventors state that alumina, silicon nitride and aluminum nitrideof their invention meet the listed properties, but do not provide anydata or examples to support any of the properties required by theinvention, for any of the composition ranges for any of the materialsdescribed.

U.S. Pat. No. 8,053,966 (Walker Jr.) discloses a method of manufacturingAl₂O₃ (alumina) ceramic spark plug insulators. In the background portionof the disclosure the inventor states (paragraph [0010]) that typicaldielectric strength (RMS) of alumina spark plug materials is about 400V/mil or 1560 V/mm, substantially lower than what is recommended for CDIin U.S. Publication No. 2010/0175655 A1 above. In the Summary of U.S.Pat. No. 8,053,966, the inventor states (Paragraph [0013] line 3) that“high purity” alumina (purity is not defined) dielectric strength can be475 V/mm or 1852 V/mil, but that this material is difficult to processand is not adequate for conventional spark-plug insulator manufacturing.None of the other properties listed as important are disclosed for anyof the claimed materials by the inventor. The inventor specificallyreferences U.S. Pat. No. 4,879,260 (Manning) and U.S. Pat. No. 7,169,723(Walker Jr.) in which the additions of Zr, Ca, Si, Mg, Ca, B oxides andBN are added to alumina ceramics in order to improve the dielectricstrength of alumina. The inventors of U.S. Pat. No. 8,053,966 and U.S.Publication No. 2010/0175655 A1 also do not recognize that the listedcombination of additives (MgO, CaO, ZrO₂ and B₂O₃ and BN) (sinteringaids) described in U.S. Publication No. 2010/0175655 A1 are noteffective in obtaining dense sintered silicon nitride or obtainingdesired properties for silicon nitride, although they may be so foralumina materials.

Sintering aids, their levels and methods of processing required foreffective sintering of silicon nitride and the combination of thesintering aids that result in desirable material properties for coronadischarge ignition systems are not given in the above mentioned priorart, and the ones discussed therein would not result in adequatematerials.

U.S. Pat. No. 5,358,912 (Freitag et al) discloses Barium AluminumSilicate in situ reinforced silicon nitride that is pressurelesssintered and contains about 3% porosity (Table 1) after sintering, whichis too high to achieve high dielectric strengths for CDI insulators.Although some of the compositions show low dielectric constants at 35GHz, values at 1 MHz (important for the CDI application) are notdisclosed. It is known that dielectric properties are especiallyfrequency dependent. Dielectric loss tangents are not disclosed.Therefore, improvements are required for materials of this invention tobe considered for a CDI application.

U.S. Publication No. 2006/0014624 A1 (Mikijelj) discloses siliconnitride compositions which result in very high dielectric strengths andlow electrical conductivity, but do not identify other properties thatneed to be satisfied in order for the said silicon nitride to be used ina corona discharge ignition system or how effective it may be in thatapplication.

From the description of the prior art above it can be seen thatmaterials of the prior art (alumina, silica, boron nitride and aluminumnitride) for the use in CDI systems are not adequate and havedeficiencies in some of their properties. Dielectric strength is low(alumina), dielectric constant and loss tangent is high (alumina andaluminum nitride e′ is 9 or above), the material is too soft (boronnitride), the material mechanical strength and fracture toughness arelow. Although silicon nitride is mentioned in the prior art in generalterms, none of the prior art recognizes the sintering aids or theircombination and amounts and ratios necessary to obtain the combinationof properties required for the CDI system to work efficiently. Thesintering aid system suggested in the prior art, in fact, does not workfor silicon nitride. What is therefore still needed is an invention thatprovides the composition ranges and defines the ceramic materialrequired for the properties to be met as well as how these materials canbe made.

SUMMARY

The present invention comprises the Si₃N₄ ceramic insulator compositionswith exceptionally high dielectric and mechanical strengths for use incorona discharge ignition (CDI) systems made from silicon nitride.Specifically, the invention comprises silicon nitride materialcompositions which, when the material is appropriately densified orsintered, provide high dielectric strength, high mechanical strength andfracture toughness, high electrical resistivity, high hardness, lowdielectric constant and loss tangent, low thermal expansion coefficientand very low porosity levels, all of which have been identified as beingimportant for the CDI system to work more efficiently than aluminaceramics described in prior art. Moreover, the materials of thisinvention retain their dielectric and structural integrity even atelevated temperature, such as above 800° C.

The invention also comprises a method of manufacture embodied in severaldistinct embodiments. In one embodiment, a sintered silicon nitride(SSN) process starting from Si₃N₄ powder batching, pressing, binderremoval and sintering is used. A second preferred embodiment of themethod of manufacture is a sintered reaction bonded silicon nitrided(SRBSN) process comprising the steps of: Si powder batching; powderpressing; binder removal; nitriding and sintering. Third and fourthembodiments are SSN and SRBSN processes respectively, in which gaspressure sintering is used for final densification. Gas pressuresintered SRBSN is the most preferred embodiment. Additional embodimentsinclude SRBSN processing in which nitriding is performed using acontinuous process according to U.S. Pat. No. 7,763,205.

It is therefore a principal object of the present invention to provide amonolithic silicon nitride compositions that exhibits high dielectricstrength, high mechanical strength, high fracture toughness, lowdielectric strength and loss tangent at 1 MHz, low porosity at roomtemperature and at elevated temperatures to approximately 800° C., thatcan be used as an insulator in a corona discharge igniter system. Thecombination of properties of the inventive Si₃N₄, allows substantiallyhigher fuel efficiency increase (from 10% to 20%) when used instead ofstate of the art alumina insulators in CDI systems. The silicon nitridecompositions of this invention are significantly different and havesignificantly better properties as compared to what the prior artrecommends for use in DCI systems. This improvement in properties isdirectly responsible for the higher fuel efficiency in a CDI system.

It is still another object of the present invention to provide severalembodiments of processes for the manufacture of monolithic siliconnitride materials above, for use in CDI systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the present invention, aswell as additional objects and advantages thereof, will be more fullyunderstood herein after as a result of a detailed description of apreferred embodiment when taken in conjunction with the followingdrawings in which:

The FIGURE is a microstructure photograph of a composition according toan embodiment of the invention herein.

DETAILED DESCRIPTION

A range of Si₃N₄ compositions in this invention have been shown toexhibit an unexpected combination of desirable properties that aresubstantially higher than commercial alumina or aluminum nitride orsilicon nitride materials of the prior art used in CDI systems.Sintering aids required to densify the silicon nitride of this inventioninclude typically a combination rare earths (Y, Ce, La, Er, Gd andothers), Al, Mg, Si (usually in form of single or oxides or othercompounds) and may optionally include small amounts of transition metalsadded as carbides, nitrides or oxides either added intentionally orcoming as impurities form the starting powders. The proportions in whichthe additives are mixed and how the material is processed affect itsphase composition and the final material properties and therefore itsusefulness in the CDI application.

In addition, it is shown that additives described in U.S. patentapplication Ser. No. 12/686,251 in the ranges described (SiO₂, ZrO₂, BN,B₂O₃, CaO, MgO) do not allow the densification of Si₃N₄ materials whenused following procedures known in the science and art of sinteringsilicon nitride materials. It is shown that entirely different sinteringaid systems are required to achieve the required properties for a CDIinsulator.

Embodiment 1

In one embodiment of the invention a dense ceramic, preferably a siliconnitride material with porosity level less than about 0.6% is provided,with the said silicon nitride having following properties:

Dielectric strength (kV/mm) >6.6 Dielectric Constant <8.3 Loss Tangent<0.006 Modulus of Rupture (MPa) >500 Fracture Toughness (MPa ·m^(1/2)) >5 TEC (10⁻⁶ 1/° C.) RT-1000° C. <3.5 Phase compositionβ-Si₃N₄ + silicon-oxynitride glass with optional minor grain boundarycrystalline phase

In this embodiment, silicon nitride is comprised of mostly β-Si₃N₄ orβ-Sialon acicular (elongated) grains and has grain boundaries that areglassy or are partially crystalline. The glassy grain-boundary is asilicon-oxy-nitride based glass containing rare earths (Y, Ce, La, Er,Lu or others), Al, Mg, Ca dissolved in the glass and optionally containssmall amounts of transition metal silicides (Fe, Mo, Ti, Cr or other) inthe form of small inclusions. The crystalline phases can be any of thephases containing the rare earths, Si, N and O, for example K, J ormelellite like phase or a rare-earth silicate phase. The overallchemical composition of the sintered Si₃N₄ of this embodiment is asfollows:

Element (wt %) Si 48-56 N 32-38 Al 0.5-9  Rare earth elements, at leastone (total) 0.5-9  Y, La, Er, Ce, Gd, Lu or other Mg  0-0.3 Fe, Mo orother transition metal, total 0.01-1   O 1-9 C 0.3-1.5

Embodiment 2

In the second embodiment of the invention a dense ceramic, preferably asilicon nitride material with porosity level less than about 0.6% isprovided, with the said silicon nitride having following properties:

Dielectric strength (kV/mm) >15 Dielectric Constant ≦8.2 Loss Tangent<0.003 Modulus of Rupture (MPa) >600 Fracture Toughness (MPa ·m^(1/2)) >5 TEC (10⁻⁶ 1/° C.) RT-1000° C. ≦3.3 Phase compositionβ-Si₃N₄ + silicon-oxynitride glass with optional minor grain boundarycrystalline phase

In this embodiment, silicon nitride is comprised of mostly β-Si₃N₄ orβ-Sialon acicular (elongated) grains and has grain boundaries that areglassy or are partially crystalline. The glassy grain-boundary is asilicon-oxy-nitride based glass containing rare earths (Y, Ce, La, Er,Lu or others), Al, Mg, Ca dissolved in the glass and optionally containssmall amounts of transition metal silicides (Fe, Mo, Ti, Cr or other) inthe form of small inclusions. The crystalline phases can be any of thephases containing the rare earths, Si, N and O, for example K, J ormelellite like phase or a rare-earth silicate phase. The overallchemical composition of the sintered Si₃N₄ of this embodiment is asfollows:

Element (wt %) Si 49-54 N 32-38 Al 2-9 Rare earth elements, at least one(total) 1-4 Y, La, Er, Ce, Gd, Lu or other Mg  0-0.3 Fe, Mo or othertransition metal, total 0.01-1   O 3-8 C 0.3-1.5

Embodiment 3

In the third, preferred embodiment of the invention a dense ceramic,preferably a silicon nitride material with porosity level less thanabout 0.5% is provided, with the said silicon nitride having followingproperties:

Dielectric strength (kV/mm) >20 Dielectric Constant ≦8.2 Loss Tangent<0.002 Modulus of Rupture (MPa) >700 Fracture Toughness (MPa ·m^(1/2)) >5 TEC (10⁻⁶ 1/° C.) RT-1000° C. ≦3.3 Phase compositionβ-Si₃N₄ + silicon-oxynitride glass with optional minor grain boundarycrystalline phase

In this embodiment, silicon nitride is comprised of mostly β-Si₃N₄ orβ-Sialon acicular (elongated) grains and has grain boundaries that areglassy or are partially crystalline. The glassy grain-boundary is asilicon-oxy-nitride based glass containing rare earths (Y, Ce, La, Er,Lu, Gd or others), Al, Mg, Ca dissolved in the glass and optionallycontains small amounts of transition metal silicides (Fe, Mo, Ti, Cr orother) in the form of small inclusions. The crystalline phases can beany of the phases containing the rare earths, Si, N and O, for exampleK, J or melellite like phase or a rare-earth silicate phase. The overallchemical composition of the sintered Si₃N₄ of this embodiment is asfollows:

Si 49-54 N 35-38 Al 2-7 Rare earth elements, at least one (total) 1-3 Y,La, Er, Ce, Gd, Lu or other Mg  0-0.3 Fe, Mo or other transition metal,total 0.01-1   O 3-6 C 0.3-1.5

Embodiment 4

In the fourth preferred embodiment of the invention a dense ceramic,preferably a silicon nitride material with porosity level less thanabout 1.5% is provided (preferably less than 1%) with the said siliconnitride having following properties:

Dielectric strength (kV/mm) >20 Dielectric Constant ≦7.7 Loss Tangent<0.002 Modulus of Rupture (MPa) >500 Fracture Toughness (MPa ·m^(1/2)) >5 TEC (10⁻⁶ 1/° C.) RT-1000° C. ≦4.2 Phase compositionβ-Si₃N₄ + hexacelsian

In this embodiment, silicon nitride is comprised of mostly β-Si₃N₄ orβ-Sialon acicular (elongated) grains and has grain boundaries that arepredominantly crystalline, containing barium-aluminum silicatehexa-celsian phase. The grain boundary phase can also contain some Srmetal, which may be a part of the crystalline phase. The overallchemical composition of the sintered Si₃N₄ of this embodiment is asfollows:

Si 34-49 N 19-31 Al 2.5-5.8 Ba  8-20 Sr 0-8 La or other rare earth 0-0.5 Fe, Mo or other transition metal, total 0.01-1   O  8-20 C0.3-1.5

CDI system insulators from silicon nitride described in embodiments 1, 2and 3 can be manufactured using several different process embodiments.

Process Embodiment 1

One embodiment for producing CDI system insulators is a sintered siliconnitride process in which fine Si₃N₄ powder with sufficiently highspecific surface area (typically 5-12 m²/g) and adequate purity is mixedwith sintering aids in the form of oxides or other compounds that willresult in oxides after thermal processing. Sintering aids includetypically a combination that includes oxides of Al, Mg, Ba, Sr, Si, Y,Er, La, Ce (and other rare earths). These can be added as oxides orother compounds which decompose or form oxides upon heating.

It is known in the art that the mixing can be accomplished in a suitablesolvent (water or organic) to which dispersants and binders can beadded. The slurry can be spray-dried, or dried in other ways to yield apowder that can be dry pressed or isopressed in a die with a suitableshape for the part desired. The part can then be green machined.

Parts can also be shaped from the mixed powder using extrusion, gel orother casting method or injection molding, the latter allowing more netshape capability for the part. After the part is shaped, it is exposedto thermal processing called binder burnout in which the organic binderis removed from the part. In the case of extrusion or injection molding,binder removal also may require dissolution of the portion of the binderin a solvent, followed by a thermal process.

After the green part without the binder is obtained, it is sintered in afurnace with a nitrogen-containing atmosphere that is oxygen free.

-   -   Sintering can be done at ambient pressure (pressureless        sintering), at peak temperatures of around 1700-2000° C.        depending on the material composition and type of furnace used.    -   Preferably gas pressure sintering can be used to densify the        parts, as this process substantially reduces the amount of        porosity in the material.    -   Hot isostatic pressing can also be used, and this can be the        most preferable process (the least amount of porosity) however        it is also the most expensive process route.

Process Embodiment 2

Preferred process embodiment for CDI insulators is the sintered reactionbonded silicon nitride (SRBSN) process because it can provide lower costinsulators. In this embodiment, instead of using silicon nitride powder,fine Si powder is used which is considerable less expensive than Si₃N₄.

As in the previous embodiment, the Si powder is mixed with the samesintering aids (quantities are adjusted based on the desired materialcomposition after Si nitriding), in the same manner. Parts can be formedand the binder removed from them in the same ways as described inprocess embodiment 1.

After the binder is removed, the parts are heated in a nitrogencontaining atmosphere (oxygen free) in the temperature range between1100 to about 1400° C. for a sufficient time to convert all the Si metalto Si₃N₄. Nitriding can be accomplished in a batch process or it can bedone in a continuous nitriding furnace as described in U.S. Pat. No.7,763,205 or similar. Continuous nitriding is a preferred processbecause it is less expensive, faster and more consistent.

After completion of nitriding, sintering or densification can beaccomplished using the same options as given before. Sintering is donein a furnace with a nitrogen-containing atmosphere that is oxygen free.

-   -   Sintering can be done at ambient pressure (pressureless        sintering), at peak temperatures of around 1700-2000° C.        depending on the material composition and type of furnace used.    -   Preferably, gas pressure sintering can be used to densify the        parts, as this process substantially reduces the amount of        porosity in the material.    -   Hot isostatic pressing can also be used, and this can be the        most preferable process (the least amount of porosity) however        it is also the most expensive process route.

Examples

Table 1 provides the starting powder compositions that were used asexamples for the materials of this invention, with the remainder beingSi₃N₄. Examples A to J are examples of this invention, and comp 1-4 arecomparative examples of the prior art described in U.S. patentapplication Ser. No. 12/686,251.

The powder mixtures according to Table 1 were made using the SRBSNprocess, starting from Si metal powder with a surface area of 1.5 to 3m²/g and purity higher that 99%, with major impurities being Fe(required for nitriding), Al and Ca, in addition to small amounts of Oon the surface. The sintering aids were added as oxides (with theexception of BN that was added as a nitride and Mo₂C as a carbide), andthe amounts were adjusted based on complete conversion of Si to Si₃N₄.Compositions A, G and N were also made starting from Si₃N₄ powderinstead of Si (powder surface area was 10.6 m²/g, purity 99%). Thesintering aid powders used have surface areas of 2 m²/g or higher. Ifthe surface area is lower, ball milling was used to increase it.

TABLE 1 Examples of comparative and inventive Si₃N₄ compositions Si₃N₄examples and compositions Example % Al₂O₃ % Y₂O₃ % CeO₂ % Er₂O₃ % La₂O₃% SiO₂ % MgO % Mo₂C B₂O₃ other A 10 2 — — — — — — — — B 16 2 — — — — — —— C 2 — 10 — — — — — — — D — — — 8 —  2.5 0.5 — — — E 5 — — — 5 — — 0.2 — — F 2 — — 5 — — — — — G 5 2 — — — —  0.25 — — — N 5 2 — — — — — 0.07 —— I 2 8 — — — — — — — — J 2 5 — — — — — 0.07 — — S — — — — — — — — — 8%LaAlO₃ T 6.5 — — — — 10.5 — — — 13.2% BaO U 5.3 — — —   0.5 10.7 — — —10.7% BaO V 10.7 —   0.3 — — 17.2 — — — 21.8% BaO comp 1 5 — — — — — — —1 — comp 2 — — — — — — 0.5 — — 2% CaO Comp 3 2 — — — — — — — — 5% BNcomp 4 — — — — — 5  — — — 0.5 ZrO₂

Powders were batched by mixing them in water or an organic solvent towhich a dispersant was added as well as a binder solution or suspension.The slurry was ball milled with alumina or Si₃N₄ (milling media) andsubsequently screened though a 325 mesh screen, and was then spray driedin a commercial spray drier, giving a free flowing powder with a medianagglomerate size of ˜120-160 μm.

Using the above powders, 4×4×0.5″ parts were pressed in a steel die to agreen density of 68-72% of theoretical. The parts were labeled, and werethen burned out at approximately 450° C. in a binder burnout furnace toremove all of the binder.

The parts that were made from Si powder were then nitrided in a standardnitriding batch furnace as well as a continuous furnace. Nitriding wasdone at peak temperature of 1400-1450° C. at the end of the process. Thebatch furnace used a nitrogen demand cycle. All parts nitridedcompletely with the exception of comp1 which had to be re-nitrided tocomplete the reaction.

All parts (including the SSN parts) were then sintered in a gas-pressuresintering furnace in a coated graphite crucible with packing powder,using a low pressure/high pressure cycle at the end of the run in orderto completely densify the compositions and eliminate remaining porosity.The sintering temperature used was between 1700 and 1950° C. Sinteringwas done in nitrogen, and the final nitrogen pressure was 15,000 psi.

After the run was completed and the furnace cooled down, the parts wereremoved and cleaned by sand blasting. Density of all of the parts wasmeasured using the Archimedes method. All of the parts of this inventionwere found to have at least 99% of theoretical density (based on itscomposition). Density was good for all SSN and SRBSN samples of thisinvention. Comparative compositions 1-4, which used the sintering aidsof the prior art in U.S. patent application Ser. No. 12/686,251, did notcompletely densify and absorbed water.

The sintered plates of this invention were made into samples for thefollowing tests:

-   -   Dielectric strength (ASTM D149 method on 0.010″ thick sample)    -   Dielectric constant and loss tangent at 1 MHz    -   Material Strength (ASTM C1161, size B bars, 4 point method)    -   Fracture toughness (ASTM C1421)    -   Thermal expansion coefficient (TEC) from RT to 1000° C.    -   X-Ray Diffractometry and phase determination of materials    -   Microstructure and porosity determination after metallographic        polishing (optical and SEM)

Table 2 lists three comparative alumina and one sintered AlN (aluminumnitride) materials that are commercially available and their properties.Alumina grades in Table 2 are used as spark plug insulators and areconsidered as state of the art.

TABLE 2 Comparative commercially available alumina and aluminum nitridematerials used in spark plugs as insulators and their properties.Dielectric Dielectric Loss Fracture Porosity TEC*, RT- ComparativeStrength constant tangent MOR Toughness level 1000° C. (10⁻⁶ PhaseExample (KV/mm) (1 MHz) (1 MHz) strength (MPa · m^(1/2)) (%) 1/° C.)composition Al₂O₃ C7000 6.6 10.1 0.010 380 3.1 0.9 8.0 α-Al₂O₃ Al₂O₃6270 4.1 9.5 0.006 350 3.0 1.3 8.1 α-Al₂O₃ Al₂O₃ UN 5.8 9.9 0.008 3303.8 1.8 8.1 α-Al₂O₃ AlN (sintered) 6.5 9 0.006 320 3.0 0.8 4.5 AlN*Thermal expansion coefficient

Table 3 has the results of property measurements made from materials ofthis invention. Results show that all of the inventive Si₃N₄compositions have dielectric strength well above the state of the artaluminas in Table 2, and Examples A, G and N have dielectric strengthabove 10 kV/mm, well above of what is reported in the prior art. Theinventive compositions demonstrate that they have other desirableproperties for the CDI insulators that are considerably better thatexpected by the prior art:

-   -   Mechanical strength is typically above 600 MPa,    -   dielectric constant is below 8.4,    -   dielectric loss tangent is below 0.003,    -   the fracture toughness is above 5 MPa·m^(1/2),    -   TEC is about 3.10⁻⁶ 1/° C. (much lower than alumina or aluminum        nitride, giving the material superior thermal shock properties)    -   Porosity level below 0.06%        all of which are desirable for the corona discharge        igniter (CDI) insulator applications.

In addition, sintered samples were chemically analyzed to determinetheir overall composition (Table 4) in addition to the phase compositionby XRD. Samples were analyzed for major constituent elements and forminor elements that are expected based on the purity of the incomingpowders or elements that come from the process contamination. It shouldbe noted that because different methods were used for the analysis, thetotals may not add up to 100%.

TABLE 3 Properties of comparative and inventive Si₃N₄ compositionsMeasured properties of examples Dielectric Dielectric Loss MOR FractureTEC*, RT- Strength constant tangent strength Toughness Porosity 1000° C.Phase Example (KV/mm) (1 MHz) (1 MHz) (MPa) (MPa · m^(1/2)) level (%)(10⁻⁶ 1/° C.) composition A 26.2 7.9 0.001 650 5.1 .02 3.3 β-Si₃N₄ +RE-Al- oxynitride glass B 17.9 7.9 0.001 600 5.0 .03 3.4 β-Si₃N₄ C 9.28.1 0.001 640 6.0 .03 3.2 β-Si₃N₄ + RE-O—N—Si phase D 10.72 8.1 0.003650 6.0 .03 3.2 β-Si₃N₄ + RE- Mg-oxynitride glass E 15.4 8.1 0.001 7005.8 .04 3.3 β-Si₃N₄ + RE-Al- oxynitride glass F 16.3 8.1 0.003 740 6.8.03 3.2 β-Si₃N₄ + RE-Al- oxynitride glass G 22.4 8.1 0.002 780 6.3 .033.2 β-Si₃N₄ + RE-Al- oxynitride glass N 20.7 7.9 0.0008 800 6.2 .02 3.1β-Si₃N₄ + RE-Al- oxynitride glass I 6.8 8.1 0.002 700 6.0 .05 3.3β-Si₃N₄ + K or J J 10.5 8.0 0.001 750 6.1 .04 3.3 β-Si₃N₄ + K or J S11.0 8.2 0.002 700 6.5 .05 3.3 β-Si₃N₄ + RE-Al- oxynitride glass T 13.07.5 0.002 520 5.5 1.0 3.4 β-Si₃N₄ + hexacelsian U 10 7.8 0.002 530 5.31.1 3.8 β-Si₃N₄ + hexacelsian V 9.9 7.1 0.003 450 4.0 1.2 4.2 β-Si₃N₄ +hexacelsian comp 1 Did not densify, absorbed water >10 comp 2 Did notdensify completely >5 comp 3 Did not densify, absorbed water >12

TABLE 4 Chemical analysis of sintered compositions Example Al Y Ce Er LaMg Mo B Ca Fe Si N C O A 5.29 1.57 — — — — 0.03 0.01 0.1 0.2 52.8 35.10.3 5.1 B 8.47 1.57 — — — — 0.02 0.01 0.2 0.3 49.2 32.7 0.2 7.9 C 1.06 —8.14 — — — 0.01 — 0.2 0.3 52.8 35.1 0.8 2.8 D 0.02 — — 7.00 — 0.30 0.01— 0.1 0.4 54.1 34.8 0.5 3.0 E 2.65 — — — 4.26 — 0.20 — 0.1 0.4 53.9 35.80.6 3.1 F 1.06 — — — 4.26 — 0.01 0.01 0.2 0.3 55.8 37.1 0.5 1.6 G 2.651.57 — — — 0.15 0.01 — 0.1 0.3 55.7 37.0 0.7 2.8 N 2.65 1.57 — — — —0.07 — 0.2 0.3 55.8 37.0 0.2 2.7 I 1.06 6.30 — — — — — — 0.1 0.2 54.035.9 0.4 2.6 J 1.06 3.94 — — — — 0.07 — 0.2 0.4 55.8 37.0 0.3 2.0 S 1.794.60 — — — 0.01 — 0.01 0.02 0.3 55.0 34.9 0.1 3.3 T 3.4 — — — — — — —11.8 0.2 44.8 27.8 0.1 12.08 Ba U 2.8 — — — 0.4  — — — — 0.1 47.1 29.40.1 10.0 V 5.7 — 0.24 — — — — — — 0.1 34.7 19.8 0.1 19.7

Inventive examples A to S in Table 3 also show that these materialsconsist of mostly crystalline β-Si₃N₄ based on XRD, and that some alsocontain minor amounts of crystalline phases residing in the grainboundary phase, surrounding the grains. Polished microstructure of thematerials (by optical and scanning electron microscopy) show that theβ-Si₃N₄ grains are elongated, and are surrounded by a grain boundaryphase that is in many cases glassy (non-crystalline). The grain boundaryphase, when analyzed by energy dispersive spectroscopy, has been foundto always contain Si, O, and N, and in addition the sintering aids usedfor making the material: Al, Y, La, Ce, Gd, Er, other rare earth oxides,Mg and so on. This phase is a silicon-oxy-nitride glass.

The FIGURE shows the scanning electron microscope microstructure ofcomposition N silicon nitride. Si₃N₄ grains are gray and elongated andthe phase surrounding the grains (lighter in color) issilicon-oxy-nitride containing dissolved Al and Y.

The Si₃N₄ material of inventive composition N was used to manufactureseveral insulators for testing in a CDI system. State of the art aluminainsulator of the same shape was tested in the same application. Testresults showed that Si₃N₄ composition N material performed better thanstate of the art alumina. The fuel economy when using the CDI systemwith the Si₃N₄ composition insulator showed about a 20% improvement,while the alumina provided about a 10% improvement. Based on the resultsin Table 3, it is reasonable to predict that compositions G or A orfurther optimized material properties of this invention would provideeven better results in the CDI system.

Inventive examples T, U, V show that the additives to silicon nitridepowder based on additions of Ba, Al and Si oxides with total additionsof about 25-50% by weight, lead to silicon nitride with a dielectricconstant lower that 7.8 which is important for the CDI system. Thisadditive system also allows small additions of rare earths that canimprove other properties, including lowering porosity level.

The comparative 1-4 examples also demonstrate that the prior artdescription of silicon nitride compositions (−251) for the coronadischarge igniter (CDI) insulator application cannot be densified whenfollowed and that they are different from this invention.

A person having skill in the science and art of making silicon nitridewill know that in addition to the rare earths listed and tested in theinventive examples, all other rare earths are good sintering aids forsilicon nitride when combined with aluminum, magnesium or siliconoxide-containing sintering aids. Therefore, use of other rare earths notspecifically mentioned does not deviate from this invention. It is alsoclear that the properties of importance for the CDI insulatorapplication have not been optimized in the listed examples and thatfurther optimization is possible without deviation from the invention.

I claim:
 1. A ceramic insulator material comprising a silicon nitrideceramic comprising β-Si₃N₄ or β-SiAlON grains and a barium-aluminumsilicate phase surrounding the grains, wherein the ceramic insulatormaterial comprises: 49-56 wt % silicon; 34-38 wt % nitrogen; 2-7 wt %aluminum; 1-5 wt % at least one rare earth element; 0-0.3 wt % at leastone of magnesium, calcium, and barium; 0.01-1 wt % transition metal; 3-6wt % oxygen; and 0.3-1.5 wt % carbon and further wherein the ceramicinsulator material has a dielectric strength of greater than 10 KV/mm, amodulus of rupture strength of at least 500 MPa, a dielectric constantof less than 8.3, a dielectric loss tangent less than 0.003 at 1 MHz, afracture toughness of at least 5.5 MPa·m^(1/2), a thermal expansioncoefficient of less than 4×10⁻⁶ 1/° C., and a porosity level less than0.06%.
 2. The ceramic insulator material of claim 1 in an internalcombustion engine.